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Objective: Head direction cell and place cell spatially tuned firing is often anchored to salient visual landmarks on the periphery of a recording environment. What is less well understood is whether structural features of an environment, such as orientation of a maze sub‐compartment or a polarizing barrier, can likewise control spatial firing. Method: We recorded from 54 head direction cells in the medial entorhinal cortex and subicular region of male Lister Hooded rats while they explored an apparatus with four parallel or four radially arranged compartments (Experiment 1). In Experiment 2, we recorded from 130 place cells (in Lister‐ and Long‐Evans Hooded rats) and 30 head direction cells with 90° rotations of a cue card and a barrier in a single environment (Experiment 2). Results: We found that head direction cells maintained a similar preferred firing direction across four separate maze compartments even when these faced different directions (Experiment 1). However, in an environment with a single compartment, we observed that both a barrier and a cue card exerted comparable amounts of stimulus control over head direction cells and place cells (Experiment 2). Conclusion: The maintenance of a stable directional orientation across maze compartments suggests that the head direction cell system has the capacity to provide a global directional reference that allows the animal to distinguish otherwise similar maze compartments based on the compartment's orientation. A barrier is, however, capable of controlling spatially tuned firing in an environment in which it is the sole polarizing feature. [ABSTRACT FROM AUTHOR]

The postsubiculum is a structure of interest because it projects to the hippocampal formation and contains head direction cells, grid cells, and border cells. The aim of the current experiment was to test whether the postsubiculum is necessary for homing by path integration. Rats were trained on a homing task on a large circular platform. After exhibiting stable homing, one group of animals (n = 6) received ibotenic acid lesions of the postsubiculum, and a second (n = 5) underwent a control surgery. After recovery, animals with postsubiculum lesions homed as accurately as the control animals. Subsequent testing on a delayed alternation T maze task showed that the lesioned animals were significantly worse than the control animals at delays of 5-, 30-, and 60-s. These findings suggest that the postsubiculum is necessary for memory and avoidance of previously visited locations but is not necessary for homing. [ABSTRACT FROM AUTHOR]

Detailed memories for unique episodes from an individual's past can be triggered, often effortlessly, when that individual is exposed to a stimulus that was present during the original event. The aim of Kesner et al. (2008) is to understand the neural basis of memory encoding that supports this cued recall of episodic memories. Kesner and colleagues make novel use of an object-place paired-associate task for rats to provide evidence for a critical role of dorsal CA3 in certain aspects of episodic memory encoding. Using one-trial cued recall versions of the task they show that when rats are cued with an object stimulus, they can be trained to revisit the location in which the object appeared previously. Conversely, when rats are cued with a location, they can learn to choose the object with which it was associated. Rats with dorsal CA3 lesions are severely impaired at these tasks. These data are consistent with the theory that the autoassociative network in CA3 supports the rapid formation of novel associations and may allow pattern completion―the phenomenon whereby a subset of the cues present at an encoding event triggers recall of the whole event. Although flexible recall of arbitrary associations is not fully demonstrated, the study contributes 2 novel behavioral tasks to the previously limited repertoire for studying paired associate recall in rats. It also builds on previous data to specify the role of the hippocampal CA3 subregion in cued recall―a critical aspect of episodic memory. [ABSTRACT FROM AUTHOR]

Path integration, the ability to maintain a representation of location and direction on the basis of internal cues, is thought to be important for navigation and the learning of spatial relationships. Representations of location and direction in the brain, such as head direction cells, grid cells, and place cells in the limbic system, are thought to underlie navigation by path integration. While this idea is generally consistent with lesion studies, the relationship between such neural activity and behavior has not been studied on a task where animals demonstrably use a path integration strategy. Here we report the development of such a task in rats: by slowly rotating rats before their return to a trial-unique home base, we could show subjects relied on internal cues only to navigate. To illustrate how this task can be combined with recording, we show examples of simultaneously recorded head direction cells in which neural activity is closely related to rats' homing direction. These results support the notion that rats can navigate by path integration, that this ability depends on head direction cells, and suggest a convenient behavioral paradigm for investigating the neural basis of navigation. [ABSTRACT FROM AUTHOR]

Abstract: Studies examining the effects of hippocampal lesions on object recognition memory in rats have produced conflicting results. The present study investigated how methodological differences and lesion size may have contributed to these discrepancies. In Experiment 1 we compared rats with complete, partial (septal) and sham hippocampal lesions on a spontaneous object recognition task, using a protocol previously reported to result in deficits following large hippocampal lesions . Rats with complete and partial hippocampal lesions were unimpaired, suggesting the hippocampus is not required for object recognition memory. However, rats with partial lesions showed relatively poor performance raising the possibility that floor effects masked a deficit on this group. In Experiment 2, we used a second spontaneous object recognition protocol similar to that used by the two other studies that have reported deficits following hippocampal lesions . Rats with complete hippocampal lesions were significantly impaired, whereas rats with partial lesions were unimpaired. However, the complete lesion group showed less object exploration during the sample phase. Thus, the apparent recognition memory deficit in Experiment 2 may be attributable to differential encoding. Together, these findings suggest that the hippocampus is not required for intact spontaneous object recognition memory. These findings suggest that levels of object exploration during the sample phase may be a critical issue, and raise the possibility that previous reports of object recognition deficits may be due to differences in object exploration rather than deficits in object recognition per se. [Copyright &y& Elsevier]